Note: Descriptions are shown in the official language in which they were submitted.
WO 93/18577 PCT/US93/01466
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METHOD AND APPARATUS FOR CORRECTING
HYSTERESIS SYNCHRONOUS MOTOR HUNTING
BACKGROUND OF THE INVENTION
The invention relates generally to hysteresis synchro-
nous motors, and in particular, to controlling hysteresis
synchronous motors used in applications which require
extremely constant rotational velocity.
Hysteresis synchronous motors can provide very constant
rotational speed when driven from a crystal oscillator based
input frequency. The operation of the motor is as follows.
A pair of quadrature drive signals, which may be either
square waves or sinusoidal, are applied to a pair of motor
windings which form the stator of the motor. The alternat-
ing stator drive currents generate a magnetic field which
rotates at the'frequency of the stator drive signals.
The rotor is composed of a solid piece of ferrous
material. The-magnetic field from the energized stator
windings induces a magnetic flux in the rotor thereby
forming corresponding magnetic poles. The rotating magnetic
field and the rotor magnetic field interact to generate a
torque that causes the rotor and motor shaft to rotate. In
response, the rotor accelerates until the angular velocity
of the rotor matches that of the rotating magnetic field.
Any angular displacement between the rotating field and
the rotor poles produces a restoring torque in the opposite
direction. In response, the rotor velocity changes to
reduce the angle thereby causing the rotor's frequency of
rotation to match the frequency of the rotating field. It
is this operation that provides the hysteresis motor with an
excellent long term stability.
Figure 1 shows a block diagram. of an electromechanical
model of a hysteresis synchronous motor. The motor can be
modeled as a second order system. More specifically, the
open loop gain of the motor is represented by equation 1
below:
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(1) G(S) -
Km
S'(J'S+B)
where Km is a motor constant, J is the motor's inertia, 8 is
a damping factor, and S is a Laplace variable. For a
typical motor, these values can be: Km = 0.049 n x m/rad;
J = 1 x 10-4 kg x m2; and B = 5 X 10-4 n x m x s/rad.
The closed loop motor transfer function is represented
by equation 2 below:
Po Km
(2) -
Pi J~S2+B~S+Km
where Pi is a Phase input, and Po is a Phase output. In a
more standard form, the closed loop motor. transfer function
is represented by equation 3 below:
Km
Po J
(3)
Pi S2+(B)~S+Km
J J
Unfortunately, the dynamic response of this type of
motor is often underdamped, due to the low damping factor B,
and the size of the inertia J. Thus, the rotor frequency
tends to drift above and below the desired frequency (i.e.,
the frequency of the stator drive signals) as the rotor
constantly attempts to match the desired frequency. For
example, a rotor which spins at a nominal rate between
150-200 rotations per second typically "hunts" at a very low
frequency of about 3 to 5 hertz around the nominal velocity.
This hunting rate is referred to as the motor's "natural
frequency" Fn and is represented by equation 4 below:
(4) wn = Km (rad/sec)
J
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A feedback servo control de~.~'_..~e is typically used to
reduce hunting and obtain an ext~:-.~.~;~ely constant rotational
velocity. Toward this end, a tac:~ ~.~,eter or shaft encoder is
~ connected to measure the instantaneous rotor frequency. A
feedback servo controller monitors the measured rotor
frequency and continuously adjusts the frequency of the
stator drive signals in an effort to maintain the rotor
frequency at a constant desired frequency.
In some applications, such a feedback servo controller
is prohibitively expensive. Accordingly, if the frequency
drift can be tolerated, a hysteresis synchronous motor is
used without feedback control. However, if very high
resolution is required, some mechanism must be used to
eliminate or reduce the frequency drift inherent in a
hysteresis synchronous motor. For example, laser scanners
used in printing devices require high precision. The
hunting of the motor produces artifacts which are easily
visible to the eye. These artifacts in the printing (or
scanning) process detract, from the image and provide a
clearly less desirable result.
It is therefore an object of the present invention to
provide a low-cost reliable, precise control mechanism to
eliminate low frequency hunting in a hysteresis synchronous
motor. Other objects of the invention area method and
apparatus employing existing equipment for controlling the
synchronous motor in a precise manner.
SUMMARY OF THE INVENTION
The invention relates to a method and apparatus for
controlling the rotational velocity of a hysteresis
synchronous motor having a low frequency hunting
characteristic. The method features the steps of generating
a nominal input phase value for controlling phase current to
the motor, the current having a nominal phase value;
repeatedly generating a measurement of the rotation time for
a single rotation of the motor; deriving a derivative phase
value from the rotation time measurements; adjusting the
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input phase value in response to the derivative phase value
to achieve a selected rotational damping phase value; and
adjusting a current applied to control the motor in response
to the adjusted input phase value for damping the low
frequency hunting of the motor.
The method further features the steps of measuring the
rotation time using a sensor responsive to energy reflected
by a mirror mounted on and rotating with the motor shaft and
using a microprocessor for adjusting the nominal phase of
the applied motor-driving current. The method further
features controlling the rotational velocity of the motor
using a microprocessor.
In another aspect, the apparatus of the invention
features circuitry for generating a nominal input phase
value for controlling phase current to the synchronous
motor, the current having a nominal phase value. Circuitry
is provided for measuring the motor rotation time, and for
deriving a derivative phase value from the rotation time
measurement of the motor. The input nominal phase value is
adjusted in response to the derivative phase value to
achieve a selected rotational damping phase value.
Circuitry is provided for adjusting the current applied to
the motor in response to the adjusted nominal phase value
for damping low frequency hunting of the motor.
Preferably, the measurement of the rotation time uses a
sensor positioned to intercept energy reflected by a mirror
rotating with and mounted on the motor shaft, and a
microprocessor is used for controlling the phase of the
applied current to achieve the selected rotational velocity.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features, and advantages of the
invention will be apparent from the following drawings in
which:
Figure 1 is a block diagram of an electromechanical
model for a hysteresis synchronous motor;
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Figure 2 is a block diagram of an electromechanical
model of a modified hysteresis synchronous motor in
accordance with the preferred embodiment of the invention;
'~ and
Figure 3 is a block diagram of the motor control
circuitry in accordance with the invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to Fig. 3, in accordance with the preferred
embodiment of the invention, a circuitry 10 controls
operation of a hysteresis synchronous motor 32. The motor
. scans the light output of a light source, for example a
laser diode, across an image plane in a straight line. The
laser diode is turned on and off as it is scanned across the
scan line using well known control techniques. In order to
lay down a sequence of equally spaced picture elements
(pels) the rotational velocity of the synchronous motor
should be constant. Small variations of the motor velocity
change the spacing between pels and cause undesirable
artifacts to appear in the finished image.
The invention includes the use of feedback to alter the
dynamic response of a hysteresis synchronous motor to
eliminate or reduce hunting.
As indicated in equations 2 and 3, the dynamic response
can be adjusted by altering the damping factor B. However,
the H term is a characteristic of the motor itself, and is
not easily increased without decreasing motor efficiency.
Therefore, modifying this parameter to improve dynamic
response is not practical.
Similarly, the dynamic response can be improved by
reducing the motor's inertia J. However, this is not easily
accomplished since the bulk of the inertia is in the rotor,
and the desired properties of the rotor constrain its
inertia.
The solution, therefore, according to the invention, is
to provide additional feedback to the motor, with the
feedback transfer function chosen so as to affect only the
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damping term of the closed loop response. Figure 2 shows a
block diagram of an electromechanical model of the motor and
an added feedback network. An analysis of this
configuration is as follows:
(S) Pe(S) _ Pi(S)_Po(S)
(6) PK(S) - Pi(S)+Pe(S)~F(S)
(7) J~S2+B~S+Km
PK(s) _ . Po(s)
Km
Substituting S into 6 and equating the result with 7 gives
J~S2+B~S+Km .
(8) ~Po(S) - Pi(S)+Pi(S)'F(S)-Po(S)'F(S)
Km
Rearranging we get
(9) Po(S)~ [J~S2+H~S+Km+Km~F(S)J - (1+F(S))'Pi(S)'Km
Which yields the following transfer function
(10) Po(S) (1+F(S))~Km
Pi(S) J.S2+B.S+Km.F(S)+Km
Letting F(S) - S'Kd where Kd is the derivative gain yields
(11) Po(S) [1+S'KdJ'Km
Pi(S) _ J.S2+(B+Km.KdJ.S+Km
Putting it into standard form gives
(12) Po (1+S~Kdl~[KmJ
- - J
Pi S2+(B+Km.KdJ~S+Km
J J
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The transfer function given by Equation 12 differs from
that of Equation 3 in only two respects. First, it has a
different coefficient in the damping term. Second it has an
additional "zero" from the S term in the numerator. It is
important to note that the natural frequency has not been
altered. The additional "zero" shows up in the numerator
only, and therefore does not affect stability of the motor
operation. The "zero" affects only the closed loop gain,
and therefore it can be compensated for elsewhere if
necessary. In this illustrated embodiment of the invention,
it is not necessary to compensate for the "zero" because the
phase input is not being modulated.
The important change is the effect on the damping term.
There is now a coefficient, kd, in that term that can be
used to modify the damping of the motor system without
affecting any other parameters.
There are several important points to note. First,
this approach is inherently different than building a servo
loop around the motor, which is the traditional approach to
solving this problem. The primary advantages of this
approach over the servo loop approach are (i) there is no
sacrifice in bandwidth, and (2) the feedback requirements
are less demanding, leading to a less expensive
implementation. A servo loop around the motor would either
have to be closed at a lower frequency than the closed loop
bandwidth of the motor itself or would require additional
compensation in order to be stable . Also a servo loop
requires accurate feedback over its entire bandwidth. The
described compensation approach only requires accurate
feedback up to the hunting frequency, so that its feedback
element can have much lower bandwidth. Because the hunting
frequency is typically very low compared to the motor's
nominal rate of rotation, it is feasible to sample the
feedback on a once per revolution basis. As noted below, in
the illustrated embodiment, this can greatly reduce the cost
of the feedback mechanism:
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ffeferring now to Figure 3, a laser recorder 30 uses a
hysteresis synchronous motor 32 to rotate a penta-prism 34
which deflects a light beam across a piece of film or other
light sensitive medium 38 which moves perpendicularly to the
scan, thus forming a raster. Just prior to entering the
image area on each revolution, the laser beam crosses a
start of line (SOL) detector 40 and thereby generates a
pulse that is used to synchronize the beam modulation for
that scan. By using the SOL 40 as the feedback element for
the motor damping, it is possible to achieve high
performance at a fraction of the cost required to servo the
motor. In this illustrated embodiment, the hunting
frequency is approximately 3 to 5 Hz, and the SOL frequency,
is approximately 175 Hz. The sampling rate (175 Hz) is
therefore sufficient to meet the feedback requirements as
described herein.
A period counter 44, incremented from a high frequency
crystal oscillator 46, counts the number of clock ticks
between successive SOL pulses. Each SOL pulse operates to
store the period counter value in an output register 48,
resets the counter 44, and interrupts a microprocessor 50.
In response to the SOL interrupt, the microprocessor reads
the counter 44 output register value and computes the
derivative of the feedback. It then multiplies this by the
derivative gain factor and stores the result as the phase
correction value. The derivative is computed using a
central difference algorithm so that the derivative gain
rolls off at high frequencies. This minimizes the
amplification of noise that might occur with a pure
differentiator, and still gives good derivative performance
at the low frequencies where the hunting occurs.
This method is slightly different than the method
indicated in Figure 2 and its associated equations.
Figure 2 indicates that F(S), the derivative operation,
should be performed on the phase error as defined in
Equation 5, which would yield the following expression.
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(13) dpe dPi dPo
dt dt dt
In this application, the input phase is constant, so that
its derivative is zero. The expression then reduces to:
A
(14) dpe dPo
dt dt
It is therefore sufficient to only take the derivative of
the feedback. No comparison to the input phase is
necessary.
The phase drive to the motor is generated by a phase
counter 60 which runs from the same crystal based source as
the period counter 44. The output of the counter 60 is
converted into a pair of quadrature waveforms which then go
into a power stage amplifier 64 which amplifies the voltage
and provides the square wave drive to the motor. The phase
counter 60 also has an associated input register 62. When
the counter overflows, it reloads from the input register,
the motor phases are switched, and an interrupt is
generated. In response to this interrupt, the
microprocessor increments an internal counter which lets the
software keep track of which phase has just switched, and
then it reloads the counter input register 62 with the
nominal phase value. When phase zero is reached (there is
an arbitrary assignment of phases 0 through 3), the
previously computed phase correction is summed with the
nominal phase modulated motor drive signal necessary to
cancel the velocity hunting.
The amount of control attainable is in part determined
by the frequency used to run the period counter and phase
counter. A typical laser scanning application requires the
phase error to remain within 30 parts per million (ppm). If
a clock rate of 32 MHz is used with the 175 Hz SOL
frequency, there results a resolution of 5.4 ppm.
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Addition, subtractions, deletions', and other
modifications of the illustrated embodiment of the invention
will be apparent to those practiced in this field and are
within the scope of the following claims.
